1. Field of the Invention
The present invention relates to a focus detection device for use in an optical disk apparatus and, more particularly, to a focus detection device for which the focus detection sensitivity can be freely set. In addition, the convergence on the data recording medium of the main optical system can be freely set during the focus detection sensitivity adjustment. It is also possible to reduce the cross-track noise of the focus error signal.
2. Description of the Prior Art
An example of conventional focus error detection method which Japanese patent laid-open number S50-78341 propose is shown in FIG. 23. The circular recording medium 1 is mounted on a rotational drive motor 4 with the motor spindle passing through a center hole 2 thereof. The reading light beam 10 emitted from a light source 5 is focused by a lens 7 on the recording track 3 formed on the surface of the circular recording medium 1. The reading light beam 10 is modulated by the recording track 3, reflected from the disk 1 through the lens 7 and off the half mirror 6, and focused on the photodetector 8. The optical system described in the above is hereinafter referred to as a "main optical system." The main optical system has a main optical axis Ap. The audio, visual, or other data recorded to the circular recording medium 1 is reproduced by demodulating the playback signal obtained by this main optical system.
This device comprises two auxiliary light sources 5a and 5b in addition to the main light source 5. These auxiliary light sources 5a and 5b are offset on opposite sides of the main light source 5 asymmetrically. The auxiliary light sources 5a and 5b are also offset to the main light source 5 with respect to the main optical axis Ap. As with the main optical system, the light beams emitted from the auxiliary light sources 5a and 5b are modulated by the recording track 3 of the recording medium 1, and are focused on the photodetectors 8a and 8b, respectively. These optical system described in the above are hereinafter referred to as "auxiliary optical systems."
In FIG. 24, the change in the modulation factor m of the playback signals caused by a given focus shift z is shown. The curves M.sub.0 ', Ma', and Mb' represent signals produced by photodetectors 8, 8a, and 8b, respectively. When the main optical system is focused on the recording medium 1, i.e., when z=0, the modulation factor m in the playback signal of the photodetector 8 is an extremely high value A', and the modulation factors m in the playback signals of the auxiliary photodetectors 8a and 8b are the same low value B' for both signals.
When the main optical system is focused at a point before the surface of the recording medium 1, i.e., when z&lt;0, the modulation factor m in the playback signal of one auxiliary photodetector 8b is the value C' which is greater than the value D in the playback signal of the other auxiliary photodetector 8b. Conversely, when the main optical system is focused at a point beyond the surface of the recording medium 1, i.e., when z&gt;0, the modulation factor m in the playback signal of the one auxiliary photodetector 8a is the value E' which is less than the value F' in the playback signal of the other auxiliary photodetector 8b.
The focus error signal can thus be obtained from the difference between the modulation factor in the playback signals from the two auxiliary photodetectors 8a and 8b, and it is also possible to correct the convergence of the lens 7.
However, the photodetection sensitivity and range in the above described method is inevitably determined by the main optical system, and cannot be readily adjusted. In particular, if the convergence of the main optical system is increased in order to read and write high density media, the convergence of the auxiliary optical system is also boosted, effectively narrowing the focus error detection range. This technology is also limited by cross-track noise, which is created when the beams move laterally across the data tracks, being mixed with the focus error signal.